Field of the Invention
The invention relates to a method for tolerance compensation in an ink jet print head, in particular in an ink jet print head having a plurality of modules on the non-interlaced principle.
Such ink jet print heads are used both in office printers and in small high-speed printers of the kind needed for postage meters and product labeling equipment. As a rule they have a relatively large number of nozzles.
One component with especially great influence on the reliability of a printer is the ink jet print head. If the ink jet print head is assembled from a plurality of components, then the precise disposition of the components relative to one another and with one another, and the ink jet print head itself, all have a definitive influence on its reliable function.
It is known, from the Third Annual European Ink Jet Printing Workshop, Oct. 16-18, 1995, in Maastricht, The Netherlands, to assemble an ink jet print head from three modules on the non-interlaced principle. The ink jet print head is typically triggered by a microprocessor.
Parallel slots of equal length, aligned with one another and extending obliquely, are made in a front panel, and the modules with their nozzle regions are inserted into them, as is also seen in FIG. 1.
A recording carrier is moved past the rows of nozzles in such a way that the printed image is composed of three strips, one above the other. Accordingly, for a vertical solid line, the upper third is created by the first module, the middle third by the second module, and the lower third by the third module.
However, it is also possible, for instance in manual postage meters, for the recording carrier to be stationary and the ink jet print head to be moved, as is seen in Published European Patent Application 0 750 277 A2.
The slots are typically made with high precision in the front panel, and the modules are manufactured as precision parts. Nevertheless, deviations in tolerance of over a tenth of a millimeter, which for a 200 dpi print head with 200 nozzles amounts to about the vertical spacing between two adjacent nozzles, cannot be excluded at the transition from one module to the next. The errors may be errors in spacing and parallelism as well as deviations from the line of alignment within one row of nozzles and in nozzles located one below another. In that respect, reference is made to a simplified example shown in FIGS. 1 and 2 and described in detail below.
A method for calibrating the nozzles of an ink jet print head in ink jet printing devices is known from Published European Patent Application 0 257 570 A2. The ink jet print head is moved bidirectionally past a recording carrier through the use of a drive mechanism. Individual droplets are each expelled from its individually triggerable writing nozzles at fixed printing times during its motion, in accordance with data taken from a drawing generator.
In terms of the method, a trial print run is first performed, during which a defined pattern of lines is printed on the recording carrier, separately for each-individual writing nozzle and each writing direction, that is leftward and rightward.
A scanning run is then carried out, during which the pattern of lines is scanned by an optical sensor disposed on the print head. The sensor is synchronized with a print cycle raster. The scanning values are transmitted, in the raster of the printed columns in a character matrix as an xe2x80x9cactualxe2x80x9d position, to a central controller of the ink writing device.
A comparison is made in the central controller, through the use of a comparison circuit, between the xe2x80x9cactualxe2x80x9d and the xe2x80x9cdesiredxe2x80x9d positions, which is determined by corresponding trigger pulses. Deviations between the two positions indicate values for a so-called droplet offset in the raster of the printed columns.
The values for the droplet offset for each writing nozzle are stored in memory separately for the two writing directions in an additional memory of the central controller. The droplet offset is sent onward, separately for the writing directions, as a distortion value to a pixel memory which is also contained in the central controller.
Each time a writing nozzle is triggered in the normal printing mode, the value for the droplet offset ascertained for the applicable writing nozzle is taken into account as a function of the writing direction. To that end, in the preparation for the characters, predistortion is performed in accordance with the printing direction and the droplet offset that is ascertained.
Accordingly, in that method, the following are required per ink writing device: one optical scanning sensor, one comparison circuit, two additional memories for droplet offset storage, and two pixel memories. That is a considerable expense for one print.
A further factor is that in ink jet print heads having a plurality of modules, tolerances from one module to another cannot be compensated for in that method. That is because a compensation is only possible chronologically beforehand or afterward along the relative line of motion of the writing nozzle.
A postage meter with an ink jet print head that has many nozzles is also known from Published European Patent Applications 0 702 334 A1 and 0 702 335 A1. The nozzles are disposed in at least two rows, which extend crosswise to the direction of advancement of the print carrier. Those two rows are also offset from one another longitudinally and transversely to the advancement direction, so that some first nozzles on the end of one row of nozzles face some second nozzles on the end of the other row of nozzles. The nozzles in the region of overlap are actuated alternatively. No correction of nozzle spacing is possible therein.
Finally, a computer-controlled ink jet printer that includes a plurality of ink jet print heads is known from German Patent DE 32 36 297 C2. The ink jet print heads are disposed one behind the other in the print carrier transport direction as well as crosswise thereto, one above the other. In that way, the printed image is created by the non-interlaced principle, which was also addressed above. Picture signals are loaded in an external memory device assigned to each individual ink jet print head. Applying the picture signals to the ink jet print heads trips an expulsion of a droplet and thus a printing operation. A time sequence and a controller are used for applying the picture signals. The picture signals are applied, in a proportion adapted chronologically to one another, to the laterally offset ink jet print heads through the use of the controller, in order to print the various lines in the desired mutual orientation. Next, a new set of picture signals for printing the next picture is stored in memory through the use of the controller.
The ink jet printer is provided with a programmable microprocessor, through which the picture signals are assembled in buffer memories assigned individually to the ink jet print heads. The storage of the picture signals in the buffers and the ensuing application thereof to the ink jet print heads are carried out through the use of a central timing and control unit with a course and fine delay device.
The latter device includes detecting the print carriers, such as the front edge thereof, at a certain point along the transport path and ensuing timing of the onset of the printing operation. The distance of the ink jet print heads from that location is known precisely, so that during the period of time between print carrier detection and the onset of the printing operation by an ink jet print head, a buffer assigned to that ink jet print head can be loaded with picture signals. The time interval can be varied through the use of the controller. In that way, the image line created by each ink jet print head on the print carrier can be shifted to the left or right in order to set the desired location of that line. That makes it possible to orient at least the first printed column.
Analogously to the device of Published European Patent Application 0 257 570 A2, an equalization of built-in tolerances of the ink jet print heads is possible, but not from nozzle to nozzle inside one ink jet print head. Spacing errors between the lines can accordingly not be corrected, either.
It is accordingly an object of the invention to provide a method for tolerance compensation in an ink jet print head, which overcomes the hereinafore-mentioned disadvantages of the heretofore-known methods of this general type, which reduces the expense for improving print quality and with which. deviations both within a module and from one module to another and built-in tolerances of the ink jet print head can all be compensated for despite the reduced expense.
With the foregoing and other objects in view there is provided, in accordance with the invention, a method for tolerance compensation in an ink jet print head, which comprises providing an ink jet print head with a plurality of obliquely disposed and mutually equidistant modules on the non-interlaced principle having first effective nozzles intended to lie on a line orthogonal to a transport direction of a print carrier; assessing test printouts, then mechanically compensating for existing gaps at transitions from one module to another by a rotation of the ink jet print head, and externally storing the gaps in memory as a correction value, before a final installation of the ink jet print head in a printing device; and then controlling the ink jet print head with a printing control computer by electronically generating individual print data as correction data for the ink jet print head and storing the print data in a nonvolatile manner within the ink jet print head.
In accordance with another mode of the invention, there is provided a method which comprises creating a first test printout with print data corresponding to a virtual straight printed line of dots extending orthogonally to the transport direction of the recording carrier; visually checking the test printout for gaps in the printed line of dots at transitional regions of the printed dots generated by the individual modules; rotating the ink jet print head about a shaft with an adjusting device, if gaps are present between applicable printed dots, for closing the gaps between the applicable printed dots; checking an outcome of a second test printout, making another correction if necessary, and mechanically storing the rotation of the adjusting device in memory by setting an adjustment screw in the adjusting device; scanning the printed line of dots with a scanner after a conclusion of a mechanical calibration, delivering scanning signals to the printing control computer and comparing the scanning signals with tripping print data in the printing control computer; storing an outcome of the comparison in memory, in the form of individual print data, in a memory integrated with the ink jet print head and constructed as a nonvolatile read-write EEPROM memory; and using the individual print data, during a printing operation, in the printing control computer to generate corrected clock signals for triggering the ink jet print head.
In accordance with a further mode of the invention, there is provided a method which comprises supplying the corrected clock signals serially and in a module-oriented manner to the ink jet print head.
In accordance with an added mode of the invention, there is provided a method which comprises supplying the corrected clock signals in the form of a parallel bus and in a nozzle-oriented manner to the ink jet print head.
In accordance with a concomitant mode of the invention, there is provided a method which comprises supplying the individual print data with information for excluding a nozzle from a printing operation, if the printed dots overlap because of the rotation of the ink jet print head.
The invention takes as its point of departure the concept of reducing the great expense for equipment and personnel, for the tolerance compensation that was previously required after an ink jet print head had been installed in a printer, by performing this compensation beforehand, externally and centrally, and implementing it in the ink jet print head.
Since individual print data for electronic calibration and installation data for mechanical calibration are stored in memory for each ink jet print head in that ink jet print head, the ink jet print head is already precalibrated before it is installed in the printer. Tolerance compensation between modules is made possible for the first time for ink jet print heads composed of individual modules on the non-interlaced principle, by combining electronic and mechanical calibration.
The auxiliaries, such as scanners and central memory devices, for the calibration, are no longer an integral component of each individual printer but instead are needed only in print head manufacture or suitable service facilities. This is a substantial economy. The expense for nonvolatile memory and an adjusting device that is needed anyway is low.
Once the ink jet print head has been installed in the printer, slight installation tolerances that may still exist can be calibrated as needed through the use of the adjusting device.
Other features which are considered as characteristic for the invention are set forth in the appended claims.
Although the invention is illustrated and described herein as embodied in a method for tolerance compensation in an ink jet print head, it is nevertheless not intended to be limited to the details shown, since various modifications and structural changes may be made therein without departing from the spirit of the invention and within the scope and range of equivalents of the claims.
The construction and method of operation of the invention, however, together with additional objects and advantages thereof will be best understood from the following description of specific embodiments when read in connection with the accompanying drawings.